CM11 Themes: Clear wallpaper mixnmatch entry on external change

If a wallpaper is applied from a theme and the wallpaper is changed
from an external source, the theme is still shown to have its
wallpaper applied.  This patch will clear this value out when a
wallpaper change is detected that is not from the ThemeService.

Change-Id: I3d32bb3c0ac96db3f940989bc7ad4eda5e4a71f7